Coccidiosis is an enteric disease of animals that afflicts domestic livestock worldwide. Businesses that rely on animal production often face significant costs because of coccidiosis, including financial losses due to the diseased livestock, as well as the expenses for the prophylactic treatments intended to reduce and/or prevent the disease. Such costs are especially relevant to the poultry industry, where intensive housing of birds favors the spread of coccidiosis.
The etiological causes of coccidiosis are members of the obligate intracellular sporozoa, subclass, Coccidia. One genus of this subclass that has significant impact on animal production is Eimeria. As is true for closely related genera Isospora, Cystoisospora, and Cryptosporidium, Eimeria requires only a single host to complete its life cycle. Under natural conditions, this life cycle begins with the ingestion of sporulated oocysts from the environment.
Eimeria are single-celled parasites with a complex, monoxenous life cycle, that exhibit a high degree of both host-species and tissue specificity. Eimeria species include those that are found in chickens: E. tenella, E. acervulina, E. maxima, E. necatrix, E. mitis, E. praecox, E. mivati and E. brunetti; and those found in turkeys: E. meleagrimitis, E. adenoeides, E. gallopavonis, E. dispersa, E. meleagridis, E. innocua, and E. subrotunda. The stages of the life cycle of Eimeria are essentially the same for all species of Eimeria, although each species has a preferred site in the intestine for development and the time required to complete the life cycle varies from species to species.
Numerous Eimeria species can infect a single host via the oral route, nasal route and/or by entry of the infectious particles into the lacrimal duct. Once ingested, the parasites penetrate the intestinal mucosal cells and undergo asexual and sexual stages of the life cycle. The resulting intestinal damage can ultimately lead to impaired growth (stunting), decreased feed utilization, loss of pigmentation, and increased mortality. In addition, the damage to the intestinal lining predisposes the animal to other infectious conditions, e.g., afflicted chickens become more prone to Clostridium perfringens-induced necrotic enteritis.
Infection begins with ingestion by a host of sporulated Eimeria oocysts. The ingested oocysts then release sporocysts in the intestine of the host. The sporocysts release sporozoites that enter intestinal epithelial cells and then transform into trophozoites. The trophozoites, in turn, undergo a process known as merogony to form first generation schizonts. Due to their relatively large size, it is the schizonts that cause the principal pathogenic effect of the infection, i.e., the tissue damage to the host.
Early generation schizonts produce numerous merozoites, which are released, and then grow and form the next-generation of schizonts. This asexual phase continues for a variable number of generations prior to the beginning of the sexual phase. The sexual phase starts when the schizonts form microgametocytes and macrogametes. The microgametocytes subsequently develop into microgametes that fertilize the macrogametes to produce unsporulated oocyst progeny. The unsporulated oocysts are then released into the intestinal lumen and excreted with the host feces. The completion of the life cycle, heralded by emergence of unsporulated oocysts in the host feces, is known as patency.
Sporulation of the oocysts occurs outside of the host, when the environmental conditions are favorable. The inevitable ingestion by a host of the sporulated oocysts begins the next cycle of infection. The time from host ingestion of the sporulated oocysts to emergence of the unsporulated oocysts in the feces is termed the prepatent time period. The prepatent time period differs among the various Eimeria species.
Poultry that are repeatedly exposed to Eimeria infections can acquire immunity from coccidiosis. In fact, depending on the immunogenicity of each Eimeria species, daily infection of broilers with small numbers of sporulated oocysts can result in the birds acquiring full immunity after as little as two repeated infections. Consequently, current protocols employing live Eimeria vaccines are based on the principle of acquired immunity, i.e., repeated infections with a small number of infective oocysts.
Vaccination generally is performed in the hatchery on the day of the bird's birth by administering the live Eimeria vaccine directly onto the birds, or through its application over their feed and/or drinking water. The infective oocysts complete their life cycle inside the intestinal tract of the bird, as described above, culminating with the release of a new generation of unsporulated oocysts in 5-11 days, depending on the species of the Eimeria. The unsporulated oocysts excreted with the feces then become infective, i.e., sporulate, in the outside environment, and re-infect the birds through host ingestion. Following two or three such cycles, the birds become immunized against coccidiosis. This immunity is characterized by: (i) a decrease and/or absence of parasites observed microscopically in the intestine, (ii) a reduction of the shedding of the oocysts, (iii) a reduction of the intestinal lesions, (iv) a reduction of the clinical disease, and/or (v) a reduction or prevention of weight lost. The acquired immunity wanes over a three to four month time period in the absence of subsequent exposure to infective oocysts.
Wild-type Eimeria are generally isolated from outbreaks of clinical disease in poultry flocks and may be propagated for use as pathogenic challenge strains. Typical non-attenuated vaccines are composed of infective oocysts from mildly to moderately pathogenic strains of the different Eimeria species that have been maintained by laboratory passage. These non-attenuated Eimeria are capable of causing coccidiosis when ingested in high numbers. Vaccine makers and users have to be careful to ensure that the vaccination provides just enough infective oocysts to elicit immunity, but not disease in the naive host. After the initial dose, the vaccination process relies solely on re-infection through the host's ingestion of sporulated oocysts from the litter.
Attenuated vaccines are made up of infective oocysts that have reduced pathogenicity. Due to the strong correlation between attenuated pathogenicity and possession of a shorter prepatent period, many attenuated strains are also precocious. Consistently, attenuated lines that possess shortened prepatent periods are commonly termed “precocious lines”.
Accordingly, some attenuation of the pathogenicity of Eimeria can be achieved through selecting for the early appearance of oocysts during repeated passage of the parasites in the host animal. In this way, populations of a given species of Eimeria have been identified that have greatly reduced prepatent time periods, and greatly reduced pathogenicity. Although the cause of the observed reduced pathogenicity is not completely understood, it is generally believed to be linked with the depletion and/or reduction in the size of at least one generation of schizonts, thereby reducing the tissue damage in the host.
There are advantages and disadvantages for both non-attenuated and attenuated vaccines. One advantage to vaccines made up of non-attenuated parasites is that the parasites replicate in larger numbers resulting in faster accumulation of oocysts in the environment, which is necessary for re-infection and subsequent immunization of the birds. On the other hand, the process of replication of non-attenuated Eimeiria in the intestinal tract of a naive chicken can produce lesions that result in poor animal welfare, loss of feed efficiency, and other detrimental effects, including secondary infections and inflammation.
Another disadvantage of non-attenuated vaccines is the necessity of ensuring that each bird receives the correct initial dose, since too large of an inoculum will cause heavy intestinal lesions, and too small an inoculum will result in a delay in onset of the immunization process, relative to the flock. In the latter case, the birds that receive an insufficient initial dose can become susceptible to being overwhelmed by the challenge due to the amplified number of infective oocysts excreted by their flock mates subsequent to the initial prepatent period. Indeed, lack of sufficient immunity prior to such subsequent Eimeria challenge(s) probably accounts for most of the failures experienced when a live non-attenuated vaccine is used.
One major advantage of attenuated vaccines is that they cause only minimal lesions. Attenuated vaccines, however, produce fewer oocysts than non-attenuated strains, resulting in slower accumulation of infective oocysts in the environment thereby, lowering the probability of re-infection following the initial prepatent period. This, in turn, results in a longer time required for the immunization to become fully established, and can even interrupt the overall immunization process. The slower accumulation of infective oocysts in the environment is particularly problematic when immunizing against the Eimeria species, E. maxima, because wild-type E. maxima produce relatively large numbers of oocysts.
Since a single host species can be infected by multiple Eimeria species, live vaccines against coccidiosis are usually designed to comprise oocysts from a number of Eimeria species. Heretofore, there have been only three types of such live vaccines: non-attenuated vaccines consisting of only laboratory cultivated oocysts; attenuated vaccines consisting of only attenuated oocysts; and mixed vaccines in which the oocysts from some Eimeria species are non-attenuated, and the oocysts from other Eimeria species are attenuated. Unfortunately, none of these vaccines overcome the disadvantages noted above. Indeed, in view of the significant disadvantages of each of the current types of live Eimeiria vaccines, along with the considerable cost to the industry of the coccidiosis due to Eimeria infections, there remains a longstanding need for improved vaccines that can better protect poultry from this costly enteric disease.
The citation of any reference herein should not be construed as an admission that such reference is available as “prior art” to the instant application.